The present invention relates to grinding machines and particularly to wood pulp grinding machines.
Grinding machines, also called "refiners", are used in the paper industry to convert wood chips into pulp wherein the wood chips are introduced between co-axially mounted rotating grinding plates having a narrow gap therebetween. Grooves in the grinding plates tear the wood fibers apart while processed wood pulp extrudes from the outer periphery of the grinding cavity between the plates. The size and shape of the cavity between the plates directly affects the quality and consistency of the wood pulp product.
Because the grinding plates are closely situated, the distance therebetween being measured in thousandths of an inch, plate gap must be carefully controlled to insure uniform pulp quality and prevent plate contact. During operation of the refiner, operating temperatures, the rate of introduction of wood chips into the grinding cavity, and plate mass distribution can affect plate gap. If plate gap is allowed to become too large, wood pulp of a coarser texture can contaminate an otherwise finely textured batch. If plate gap becomes too small, the plates may clash in undesirable metal to metal contact. Thus, a constant plate gap should be maintained throughout refiner operation. Similar considerations hold for plate tram or plate parallelism; the plates should remain parallel during refiner operation.
Due to great pressure exerted upon the plates during refiner operation, it is possible for the outer periphery of a grinding plate to actually bend or move away from the plate cavity while the interior portions of the plate remain substantially in place. This condition, termed deflection, may be determined by detecting a greater gap around the periphery than that is found nearer the interior of the cavity. Dynamic measurement of plate deflection is an important system capability.
Plate wear should also be monitored so as to allow optimal replacement of grinding plates. Without a means for measuring wear during operation of the refiner, the refiner has to be shut-down just to check wear. Dynamic measurement of plate wear, i.e., during refiner operation, would have the advantage of requiring at most one shut-down to replace worn plates; in cases where plate replacement can be coordinated with other refiner shutdowns, efficiency is thereby enhanced.
In sum, four measurements are important to the operation of wood chip refiners: gap, tram, deflection and wear. It is desirable to monitor and control these parameters during refiner operation to provide uniform pulp quality, prevent metal to metal contact, and to allow optimum replacement of worn plates. Various measurement devices have heretofore been developed for measuring gap, tram, or wear, and in some cases a single device measures both gap and tram. However, no single apparatus is known, preceding the present invention, which provides means for dynamic measurement of gap, tram, deflection and wear.
May Pat. Nos. 3,500,179 and 3,434,670 disclose the use of a magnet located at the periphery of one grinding plate and a series of coils located at the periphery of the other grinding plate, opposite the magnet. During relative rotation of the two plates, the magnitude of signals developed in the coils caused by the magnetic field of the passing magnet, are representative of the local distance between the grinding plates. This method is limited to distance measurements at the periphery of the grinding plates and does not include measurement of plate wear.
Garr Pat. No. 2,548,599 teaches the use of a first inductive sensor embedded in a grinding plate and directed toward the grinding surface of the opposing grinding plate; the output from the first inductive sensor is representative of the distance to the opposite grinding surface. The sensor is mounted flush with the grinding surface and unfortunately wears as the surface of the grinding plate wears. A second sensor is used to generate a control signal wherein the second sensor is stationary and directed toward a stationary metallic surface. The control distance from the second sensor to the stationary metallic surface can be accurately measured and controlled by means of independent measurement using micrometers. The outputs of the first and second sensors are compared, while the control distance from the second sensor to the stationary metallic surface is manipulated. At a time when the sensor outputs are equal, the control distance from the second sensor to the stationary metallic surface is equal to the gap between the grinding plates. Although this method provides distance measurements within the periphery of the grinding plates, the sensor becomes worn during operation and as a result must be replaced. Furthermore, no means for measuring plate wear is disclosed.
An inductive sensor device is disclosed in Akerblom Pat. No. 4,387,339. A core element of a material highly permeable to magnetic action is located within one grinding plate. Disposed about the core are two windings, one of which is closer to the grinding surface of the other grinding plate. Each of said windings is supplied with current so as to energize the windings in opposite directions and this current is controlled for maintaining a resultant magnetic flux through a DC field meter positioned between the windings equal to zero. The difference between the currents supplied to the windings is representative of the distance between the grinding plates. This method is limited to the measurement of the gap and tram between the grinding plates.
In the past, sensors have necessarily been mounted flush with the grinding surface of the plate in which they are mounted, and, as a result, such sensors experience wear during refiner operation. Gap readings taken from a recessed sensor would be inaccurate unless plate wear were considered, a parameter heretofore not dynamically measurable. What is needed is an apparatus for measuring plate wear and which would then allow recessed sensors to accurately measure the gap between plates.